1. Field of the Invention
The present invention relates to a lithographic apparatus and a method of using the apparatus in the manufacture of a device such as an integrated circuit (IC). In particular, the present invention relates to a lithographic apparatus designed to be used with radiation having a wavelength in the Extreme Ultra-Violet (EUV) range and wherein the lithographic apparatus is designed to provide a radiation beam with enhanced spectral purity.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that use, a patterning device, which may alternatively be referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several, dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction, the “scanning” direction, while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Between the reticle and the substrate is disposed a projection system for imaging the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of irradiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
An important feature in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. A number of parameters affect the available resolution of features, and one of these features is the wavelength of the radiation used to expose the pattern.
It is anticipated that the use of EUV lithography will enable the manufacture of feature sizes down to 32 mm using radiation with an EUV wavelength between 5 and 20 nm, and typically 13.5 nm. Radiation at this wavelength is absorbed in all materials and is therefore not suitable for use with refractive optics. The optics in a projection system for use with EUV lithography must therefore be based on reflective elements, for example mirrors, which can only operate in an Ultra-High-Vacuum (UHV) environment. The projection system is therefore enclosed in a Projection Optics Box (POB) which is kept under a vacuum.
However, a significant difficulty in EUV lithography is designing a system which has a projection beam with high spectral purity.
For example, some EUV sources, especially plasma sources, emit radiation over a wide range of frequencies, even including infra red (IR) visible (VIS), ultra-violet (UV) and deep ultra-violet (DUV). These undesired frequencies will propagate and cause heating problems in the illumination and projection systems and cause undesired exposure of the resist if not blocked. In addition, although the multi-layered mirrors of illumination and projection systems are optimized for reflection of the desired wavelength, e.g. 13.5 nm, they are optically flat and have quite high reflectivities at IR, VIS and UV wavelengths. It is therefore necessary to select from the source a relatively narrow band of frequencies for the beam of radiation. Even where the source has a relatively narrow emission line which contains a large part of the energy, it is necessary to reject radiation out of that line, especially at longer wavelengths. Previously, it has been proposed to use a thin membrane as a filter to perform this function. However, such a film is very delicate and becomes very hot, 200-300° C. or more, leading to high thermal stresses and cracking, sublimation and oxidation in the high power levels necessary in a lithographic projection apparatus. A membrane filter also generally absorbs at least 50% of the desired radiation.
Furthermore, U.S. Pat. No. 6,678,037, incorporated herein by reference, describes a lithographic projection apparatus wherein a grating spectral filter is used in the radiation system of the lithographic projection apparatus. The grating spectral filter in U.S. Pat. No. 6,678,037 is designed for allowing radiation of desired wavelengths to form a beam of radiation and for deflecting radiation of undesired wavelengths. The grating spectral filter is substantially formed of a material having a complex refractive index close to unity at the desired wavelengths and includes silicon protrusions. The protrusions have a laminar sawtooth profile or a laminar square wave profile and are present on a mirror, having a mirroring surface. Additionally, the filter in U.S. Pat. No. 6,678,037 is silicon and does not block IR radiation very well.